Now that you have an idea of where your OS project is heading as a whole, it's time to go into specifics. The first component of your OS which you'll have to design, if you're building it from the ground up, is its kernel, so this article aims at being a quick guide to kernel design, describing the major areas which you'll have to think about and guiding you to places where you can find more information on the subject.

I think you forgot the SymbianOS. It might be worth to mention it since it has some interesting concepts, even though it is a bit unclear for how long it will exist (and for how long you can look at the source code to learn from it)